[cmake] Move HandleOutOfTreeLLVM to common cmake utils

This is better than libunwind and libcxxabi fishing it out of libcxx's
module directory.

It is done in prepartion for a better version of D117537 which deduplicates
CMake logic instead of just renaming to avoid a name clash.

Reviewed By: phosek, #libunwind, #libc_abi, Ericson2314

Differential Revision: https://reviews.llvm.org/D117617

cmake/Modules/HandleOutOfTreeLLVM.cmake

@@ -1,5 +1,5 @@
 if (NOT DEFINED LLVM_PATH)
-  set(LLVM_PATH ${CMAKE_CURRENT_LIST_DIR}/../../../llvm CACHE PATH "" FORCE)
+  set(LLVM_PATH ${CMAKE_CURRENT_LIST_DIR}/../../llvm CACHE PATH "" FORCE)
 endif()
 
 if(NOT IS_DIRECTORY ${LLVM_PATH})

libcxxabi/CMakeLists.txt

@@ -37,10 +37,6 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBCXXABI_STANDALONE_B
   set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
   set(PACKAGE_BUGREPORT "llvm-bugs@lists.llvm.org")
 
-  # Add the CMake module path of libcxx so we can reuse HandleOutOfTreeLLVM.cmake
-  set(LIBCXXABI_LIBCXX_CMAKE_PATH "${LIBCXXABI_LIBCXX_PATH}/cmake/Modules")
-  list(APPEND CMAKE_MODULE_PATH "${LIBCXXABI_LIBCXX_CMAKE_PATH}")
-
   # In a standalone build, we don't have llvm to automatically generate the
   # llvm-lit script for us.  So we need to provide an explicit directory that
   # the configurator should write the script into.

libunwind/CMakeLists.txt

@@ -37,10 +37,6 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBUNWIND_STANDALONE_B
   set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
   set(PACKAGE_BUGREPORT "llvm-bugs@lists.llvm.org")
 
-  # Add the CMake module path of libcxx so we can reuse HandleOutOfTreeLLVM.cmake
-  set(LIBUNWIND_LIBCXX_CMAKE_PATH "${LIBUNWIND_LIBCXX_PATH}/cmake/Modules")
-  list(APPEND CMAKE_MODULE_PATH "${LIBUNWIND_LIBCXX_CMAKE_PATH}")
-
   # In a standalone build, we don't have llvm to automatically generate the
   # llvm-lit script for us.  So we need to provide an explicit directory that
   # the configurator should write the script into.